Archive for the ‘Electronics’ Category

One of the long term aims of the weather station is to take the system off grid. This will mean using an alternative energy source, something other than mains electricity. This usually means using solar energy to charge batteries in order to keep the system running.

Another scenario that will need to be considered is the case where the weather station is collecting data but is unable to connect to the internet. In this case it is desirable for the weather station to continue to collect data, record the date and time the observation was made and also store the readings. The system can then upload the data when the network connection is once available once more.

This article presents a possible solution to one problem and a possible solution to another:

Reducing power by putting the system to sleep

Recording the date / time when the system is not connected to the Internet

The solution being considered is a Real Time Clock (RTC) module based upon the DS3234 chip.

DS3234 RTC

The DS3234 is a real time clock module with a built in temperature controlled oscillator. It is a relatively self-contained module requiring few external components. The chip is programmed over the built in SPI interface.

A summary of the important features being considered are:

Built in alarm(s)

Battery back up

Built in oscillator

Interrupt on alarm

One additional feature that may be useful is the provision of 256 bytes of battery backed up RAM.

The interrupt generated by the alarms can be used to trigger the Oak to take action. In this way it may be possible for the Oak to sleep for a period of time waiting for a signal from the RTC. The RTC will run in much lower power mode than the Oak.

One disadvantage of this module, at the time of writing, was the lack of code that would either provide the functioality required or work reliably on the Oak.

Time to break out the C++ compiler.

Controlling the DS3234

Before we look at the software, it is worth spending a little time to review how the DS3234 is controlled. The RTC chip uses write operations to a series of registers in order to control the functionality of the chip. The register map is as follows:

Register Read Address

Contents

0x00

Current time seconds

0x01

Current time minutes

0x02

Current time hour

0x03

Current time day

0x04

Current time date

0x05

Current time month

0x06

Current time year

0x07

Alarm 1 seconds

0x08

Alarm 1 minutes

0x09

Alarm 1 hour

0x0a

Alarm 1 day/date

0x0b

Alarm 2 minutes

0x0c

Alarm 2 hour

0x0d

Alarm 2 day/date

0x0e

Control register

0x0f

Control / status register

0x10

Crystal aging offset

0x11

Temperature (MSB)

0x12

Temperature (LSB)

0x13

Disable temperature conversions

0x14

Reserved

0x15

Reserved

0x16

Reserved

0x17

Reserved

0x18

SRAM address

0x19

SRAM data

Applications can write to the registers by setting bit 7 (i.e. add 0x80) in the address.

The register address is automatically incremented after a read or write operation. So to write the current time into the hours, minutes and seconds registers an application would simply write four bytes to the SPI bus; namely address 0x80 (write address for the seconds register) followed by the seconds, minutes and hour values.

The bit fields for the above registers are well defined in the DS3234 datasheet and are not covered in too much detail here and you are encouraged to download the datasheet for reference.

DS3234 Software – Private Interface

The full source code for this module can be downloaded from at the end of this article, the discussion about the software and it’s functionality will concentrate on the class definition in the header file. Those interested in the detail can jump into the source code at the end of the article.

void SetRegisterValue(const Registers reg, const uint8_t value)

As noted in the hardware discussion, the DS3234 will automatically increment the “address register” if multiple bytes of data are written to the SPI bus. These two methods take advantage of this functionality.

The first variant of the overloaded method, BurstTransfer(uint8_t *dataToChip, uint8_t size), transfers a number of bytes defined by size to the SPI bus. No data is read from the bus.

RateSelect

It is possible to generate a square wave at one of four frequencies on the interrupt pin. These flags determine the frequency of the generated signal:

enum RateSelect { OneHz = 0, OnekHz = 1, FourkHz = 2, EightkHz = 3 };

Registers

There are a total of 25 register addresses on the DS3234, containing information about the time, alarms and various other functions of the DS3234. This enumeration contains the addresses of several of the registers excluding the time register addresses:

Constructor and Destructor

As already noted, the default constructor has been made private but two public constructors have been created. This ensures that the developer is forced to define one or more of the parameters for the SPI bus.

DS3234RealTimeClock(const uint8_t chipSelect)

The only parameter to this constructor is the pin to be used for chip select.

If this constructor is used the default pins are used for MOSI, MISO and SCLK.

setup‘s first job is to clear any interrupts and set the DS3234 interrupt pin to generate an interrupt.

The next block of code which is commented out, allows the application to set the current date and time. This is only required when the application is first run or if the application is run with a DS3234 without a battery installed. If a battery is not installed then it may be desirable to get the current date and time for a time server if one is available.

The next step is to get the current date and time from the module and add two minute to the time, set the seconds for the alarm to 0 and then set an alarm for two minutes time. The first time this is run may generate an alarm somewhere between 1 and 2 minutes time. All subsequent alarms should occur at two minute intervals.

Setting the seconds for the alarm to 0 makes the first alarm appear at a seemingly random time. Consider the case when the application starts at 13:46:32. The application will set the alarm time to 13:48:00, i.e in 1 minute and 28 time and not 2 minutes. The following alarm will occur 2 minutes after the current alarm.

Finally, setup will set the interrupt behaviour on pin 5 of the microcontroller to trigger on the falling edge.

All of the work is performed in the setup method and the interrupt handler. This means that the main program loop does not contain any code:

void loop()
{
}

All of the work for this application is performed in the interrupt handler:

RTCinterupt is invoked then the interrupt pin of the DS3234 falls from high to low. This will happen when the specified alarm event occurs, in this case when the minutes and seconds of the current date / time match the value in Alarm1.

First thing to do is to clear the current interrupt taking the interrupt line from low back to high.

An interrupt counter is being stored in the battery backed up memory. This is read, incremented, displayed and then written back into the SRAM.

Finally, the alarm is reset to occur in two minutes.

Conclusion

DS3234 modules provide a method of recording time when the Internet is not available. The intention is to provide a way of taking readings and timestamping the readings when Internet connectivity is not available. Data can be uploaded when connectivity is restored and the time information will be retained.

The SRAM could be used to store readings along with a timestamp however, with only 256 bytes available this would not allow for a lot of readings and so another method will be investigated.

A previous post looked at measuring rainfall using a pluviometer that generated a pulse for every 0.2794mm of rainfall. The conclusion of that post was that the algorithm used to read the counter could generate a significant error in the right circumstances.

So let’s have a look at what we can do about the error.

Parallelism

Time for an admission, the algorithm used to collect the sensor reading was not as efficient as it could have been. The code read one bit at a time from the MCP23017 chip and built up the 8-bit sensor reading gradually. This reading took approximately 2800us to complete.

This is more than enough time for one or more of the bits to change in the time taken to take the sensor reading.

Fix One – Read 8 Bits in One Go

A more detailed examination of the library for the MCP23-17 reveals methods for reading not only one bit but 8 and 16 bits in a single operation (from a callers pint of view). The point about a callers point of view will become important later.

For a single 8-bit read the application can obtain all of the bits in a single, near instantaneous pass. Reading the counter in this manner takes approximately 480us. Most of this time will be spent setting up the call to the API and talking to the MCP23017 at 100kHz.

So for a single 8-bit read we have reduced the possibility or error caused by the data changing during the time the reading is being made.

Fix Two – Stop the Clock – FAIL

Within the API for the MCP23017 is an additional method for reading both of the registers “at the same time”. This is quoted because the library reads the first 8 bits in one request and then makes a second request for the next 8 bits. The request mechanism is working at 100 kHz and so there is going to be a finite delay between reading the two 8 bit registers. Plenty of time for the values to change.

One solution is to borrow on an old idea from CPU design, namely using a clock to synchronise operations.

Any operation in a CPU is a combination of signals passing from one gate to another, to yet another and so on. Each gate takes a finite amount of time to process any signals and generate a result. This time is know as the propagation delay.

In the case of the pluviometer, the propagation delay is the amount of time needed to read the counter (the registers in the MCP23017).

In a CPU the clock synchronises the signals and output by effectively freezing the state of the system. So, the input to a system might be changing but the use of the clock freezes the output of the CPU until the next clock pulse.

The same idea can be used with the pluviometer but instead of freezing the output we freeze the input while a reading is being made.

The only problem with the implementation being considered was that the switch caused multiple signals to be generated even with the debounce circuit.

Time for a rethink.

Fix Three (or Two Revisited) – Multiple Readings

Stepping away from a problem can often yield an alternative solution as the mind works on the problem subconsciously. The problem is that we could have a reading taken whilst the input signal is changing the value being read. So if a reading is taken twice and the values are the same then we can be confident that the input has not changed provided that the time between the samples is longer then the time taken for the input to change.

A simple solution to the problem.

Software Changes

The software change is minimal, read the input twice until a consistent sample is read, then reset the counter.

A retry counter is also used in order to prevent the reading method from blocking the application.

One important point to note is that if the retry count exceeds three, then the method exist but sets the current count to zero without resetting the pluviometer counter (hardware). This allows the rainfall measurement to continue and allow for the reading not to be distorted.

Conclusion

A couple of minor software changes have improved the reliability of the pluviometer samples. There is still the chance of missing a single sample (0.2794mm) for every time the counter is read.

A further enhancement would be to increase the time between samples. This could be done by taking a measurement every hour (or more) or to accumulate the readings over a long time (say a day) and reset the counter once a day.

The last days have seen the final two sensors connected to the Oak, the wind direction and the ground temperature sensors.

Ground Temperature Sensor

This sensor is a DS18B20 sensor that has been encapsulated in a rubber like coating to make it waterproof. The sensor has a 1.8m (6 ft) waterproofed cable attached.

DS18B20 Waterproof Temperature Sensor

The sensor uses a one wire protocol to communicate with the microcontroller, so only three connections required, power, data and ground. Strictly speaking, only two connections are required, data and ground as the sensor can use power from the data line to provide enough power for the sensor.

Adafruit also provide a library for communicating with the DS18B20 sensor along with some example code. This makes working with the sensor a relatively simple job. Most of the code required to set up and read the sensor was taken from the example code provided and as such will not be discussed is any detail here.

Wind Direction Sensor

The wind direction sensor is an analogue sensor and the last of the sensors on the Weather Station sensor kit.

The sensor consists of a series of reed switches in a star shape. The wind vane uses magnets to close the reed switches and the resulting connections connect a resistors to ground through one of the wires. The second wire should be connected to a a resistor (the example in the data sheet is 10K) with the other end connected to Vcc.

The result is a potential divider circuit. The output from the circuit can be connected to an Analog to Digital Converter (ADC). The datasheet for the sensor is supplied with a list of the resistance values along with expected outputs for a 5V supply. All of the circuitry used so far has been using a 3.3V supply as the Oak is a 3.3V component. A little bit of spreadsheet wizardry converts the values to a 3.3V supply:

Degrees (from North)

Resistance

Reading (5V)

Reading (3.3V)

0.0

33000

3.837

2.533

22.5

6570

1.982

1.308

45.0

8200

2.253

1.487

67.5

891

0.409

0.270

90.0

1000

0.455

0.300

112.5

688

0.322

0.212

135.0

2200

0.902

0.595

157.5

1410

0.618

0.408

180.0

3900

1.403

0.926

202.5

3140

1.195

0.789

225.0

16000

3.077

2.031

247.5

14120

2.927

1.932

270.0

120000

4.615

3.046

292.5

42120

4.041

2.667

315.0

64900

4.332

2.859

337.5

21880

3.432

2.265

This now brings us on to the problem of the number of ADC channels on the Oak, there is only one and we have two analogue devices, the ultraviolet light and the wind direction sensors. One solution is to add a multichannel ADC to the system.

The ADS1115 is a four channel ADC with a an I2C bus. As with the DS18B20, there is a library available in the Arduino library collection, again written by Adafruit.

In the initial experiments with this library using a potentiometer I experienced problems with the conversions appearing to be duplicated / incorrect. It appeared that the conversion was appearing on the next channel (i.e. conversion for A0 was given on A1) or on both channels (i.e. A2 and A3 having the same reading). Perhaps there is not enough time being left for the ADC to complete the sample and conversion. A brief investigation into the library code lead to a pause parameter being used to allow the ADC to perform the conversion. This was 8ms, changing this to 10ms fixed the issue.

Next step is to calibrate the wind direction sensor and confirm the sensor readings with the theoretical values from the spreadsheet.

A painstaking few hours with the sensor, the ADC and the spreadsheet followed. The net result was the following:

Degrees (from North)

Resistance

Theoretical Reading (V)

Actual Reading (V)

Actual Reading

0.0

33000

2.533

2.4913

13287

22.5

6570

1.308

1.2873

6866

45.0

8200

1.487

1.4628

7801

67.5

891

0.270

0.2647

1412

90.0

1000

0.300

0.2938

1567

112.5

688

0.212

0.2084

1112

135.0

2200

0.595

0.5857

2134

157.5

1410

0.408

0.4004

2136

180.0

3900

0.926

0.9106

4587

202.5

3140

0.789

0.7745

4131

225.0

16000

2.031

1.9948

10639

247.5

14120

1.932

1.9953

10642

270.0

120000

3.046

2.9958

15978

292.5

42120

2.667

2.6233

13991

315.0

64900

2.859

2.8132

15004

337.5

21880

2.265

2.2282

11884

The final column in the table above is the actual 16-bit reading from the ADC. The intention is to use these values when working out the wind direction, it is quicker (from a processor point of view) to work with integers rather than floating point numbers. Comparisons are also more accurate.

From a code perspective, the best way to deal with the above is to use a lookup table. The table can be ordered on the actual reading, or a mid-point reading.

If the readings are ordered in ascending order then we can calculate the values that are midway between two readings. The idea is to give a spread of possible values for a particular reading. There will be slight variations in the ADC reading around the theoretical value for any particular angle. Rather than define the actual spread, or a tolerance, say 100 points either way, a mid-point value should give a very wide range and should ensure that we have the right result for the particular ADC reading.

So let’s look at some code. There are a couple of basic things we need to set up, an enumeration to represent the wind direction and a data structure for the look up table:

The last two entries direction and directionAsText are really there to give us a meaningful description within the code and also a textural description should we need to output the wind direction to a display.

The principle is to walk through the look up table and compare the reading from the ADC with the midpoint reading. The midpoint for any element in the table is the value halfway between this value and the previous value.

So if a reading x is greater then xm and greater than xm+1 (where xm is the midpoint of the current element in the table xm+1 is the midpoint of the next element in the table) then the program should move on to the next element.

If in doubt walk thorough an example. Take a reading x = 1567, the exact reading for East and walk through the table above. The answer you should reach is East. Now take a reading x = 1566 or x = 1568 and the answer should be the same, namely East. Finally, be a little more extreme, say x = 1530, the answer should still be the same.

Conclusion

The final two sensors have been connected to the Oak (on breadboard) and indoor readings are available through the serial port.

Next steps:

Fix the rain fall meter issue wher it is possible to produce an incorrect reading when a pulse is generated by the rainfall guage at the same time as the gus=age is being read.

Add a real time clock (RTC) to the system for times when the internet is not available.

Send data to the cloud

Transfer the design to protoboard and start to work on PCB designs

Still plenty to do on the design / coding front. And at some point this needs to be made hardy enough for the great outdoors, and become self powered.

The pluviometer is a mechanical sensor that looks a little like a see-saw. A small plastic bucket is placed on each side of the fulcrum. The buckets are designed so that when a critical mass of water is in one bucket it tips the see-saw and the bucket on the other side starts filling. The process continues as each bucket is filled and emptied. A picture paints a thousand words, so here is a view of the inside of the pluviometer:

Inside the Pluviometer

A small magnet is placed on the see-saw and a reed switch is behind the fulcrum. The tipping motion triggers the reed switch each time the see-saw tips, this in turn generates a single pulse.

Reading the switch becomes a seeming simple repetition of the wind speed problem, namely debouncing a switch and attaching an interrupt.

One long term goal has to take this solution off grid. This will make power consumption a critical factor in the design. Attaching an interrupt does not necessarily become an attractive option as the Oak would be running continuously and with the WiFi running this would consume a fair amount of power.

According to the specification for the pluviometer, a pulse will be generated for every 0.2794mm of rainfall.

Offline Counting

One possible solution to the power problem would be to put the Oak to sleep and wake it up every say 5-15 minutes to take measurements and upload them to the cloud. Doing this would reduce power but would also mean that no measurements would be take during the sleep period if interrupts were used.

A cheap solution in terms of cost is to use the 74HC4040 counter. This could be put into a circuit and kept active while the Oak is sleeping. The output from the debounced switch would then be used as a clock signal for the 74HC4040. This would allow the pulses from the pluviometer to be counted while the Oak is sleeping.

The downside is that eight pins are needed to read the output from the counter. With only a small number of pins, many of which have already been used, this will require some way of adding extra pins to the system. Fortunately there are a number of digital IO expanders on the market. A common series of chips is the MCP23x17 chips. These add an additional 16 inputs/output pins with communication to the chip vis I2C (X=0) or SPI (X=S). The I2S variation will fit the bill nicely.

One final connection that is required for the counter is a reset connection. Left to itself, the rain gauge counter would continue to count pulses until the counter overflowed and started from zero once more. From a design perspective there are a few options:

Reset the counter each time it has been read

Reset the counter once a day

Let the counter overflow and detect the reset to zero

One consideration will be the amount of rainfall the system can detect before it resets to zero.

Maximum number of counts = 212

Which gives 4096 counts. However, for simplicity we will only consider the lower 8 bits, i.e. 0-255 counts. The system can always be expanded later should this be necessary (or if the system has the capacity).

Maximum rainfall = 255 * 0.2794 mm

Giving a maximum rainfall count of 71.247 mm per counting period. It is envisaged that the counting period would be somewhere in the region of 5-15 minutes. This would make an hourly average of 285mm assuming a steady rainfall and a 15-minute interval between counts.

This should be well within expectations for UK weather.

As an aside, the full 12-bits would allow for a rainfall of 1144mm per counting interval, or 4.5 metres of rain per hour.

Schematic

With a little supporting hardware to be added, the schematic looks as follows:

Weather Station Partial Schematic

Note that the BME280 and TSL2561 are also on the schematic.

The rain gauge counter bit values have been labeled RXQx, RG = Rain Gauge and Q is the standard notation for a bit in a logic design.

Software

The MCP23017 (I2C version of the IO expander) is sold by Adafruit. They have a standard Ardunio library for this component and so in the interest of code re-use, expediency and idleness this will be used.

So the first thing to do is to create an instance of the MCP23017 IO expander:

Next up, the output expander needs to be setup. Looking at the schematic, the output from the counter is mapped to port B on the MCP23017. This maps to IO pins 8 to 15 inclusive. These bits should be set to inputs. One final piece of configuration is the reset pin, this needs to be set to output:

Accuracy

The eagle-eyed amongst you will have spotted a small flaw in the above logic, namely that the pulse counter could increment whilst the system is reading the values. It turns out that this matters a great deal in some cases. Consider the following:

The pulse counter has counter 15 pulses (0b00001111). The software is processing these bit values and has processed bit 2, counter value = 7. A pulse is registered by the counter and the pulse count becomes 16 (0b00010000). The system processes bit 3 and finds it is 0 and then moves on to bit 4 and finds this is 1, i.e. 16. The counter then becomes 16 + 7 = 23 instead of 15. A little bit difficult to see, so lets put this in tabular form:

The problem occurs because the reading of the pulses is not instantaneous, the software takes longer to process the data than the counter takes to register a clock pulse and update the counter.

There are two possible solutions to this, both left for later investigation:

Add an AND gate between the incoming rain gauge pulse and the 74HC4040 clock pin. This can be turned off when taking a reading.

Check the documentation for the MCP23017 to see if the values can be latched and then read. Latching the values means that they cannot be changed; once latched and the values being read would be consistent.

Both offer a solution to the problem and will be left as a later refinement.

Conclusion

The pluviometer is a seemingly simple switch but it needs to run constantly. This presents a challenge when considering the requirement to save power where possible. This article presented a possible solution using a small number of components bringing powered on permanently.

The solution does introduce a possible error in the reading, although this should be reasonably small. A full analysis will be presented in a future article.

The weather station project is still at the proof of concept stage but the last few articles have run through the concepts for connecting several sensors to the Digistump Oak microcontroller. In this article we will look at the basic (and I stress basic) software required to connect the sensors discussed so far with the microcontroller and start to collect data.

Hardware – So Far

These sensors are currently connected to a piece of breadboard along with the Oak.

Now we have the sensors connected we need to add some software goodness.

Software Requirements

The first and most obvious requirement is to be able to collect data from the above sensors. In addition the proof of concept software should also permit the following:

Test logging data to the cloud

Serial debugging

Setting the system time from the Internet

Data logging to the cloud will initially be to the Sparkfun Data Service as this is a simple enough service to use. One of the first things to do is to create an account / data stream in order to permit this.

Libraries

In previous articles it was noted that Sparkfun and Adafruit provide libraries for two of the I2C sensor boards being used (BME280 and TSL2561). An additional library is also required to support the third objective, setting the time from the Internet.

The additional libraries are installed from the Sketch -> Include Library -> Manage Libraries.. dialog. Open this dialog and install the following libraries:

Adafruit Unified Sensor (1.0.2)

Adafruit BME280 (1.0.3)

Sparkfun TSL2561 (1.1.0)

NtpClientLib by German Martin (1.3.0)

The version numbers are the ones available at the time of writing.

Software Walk Through

At this point the hardware should be in place and all of the necessary libraries installed. Let the coding begin.

This first thing we need is the includes for the libraries that are going to be used:

Before we progress much further it should be acknowledged that some of the code for the BMS280 and the TSL2561 is modified from the Adafruit and Sparkfun example applications.

A key point to note from the above code is the definition of the windSpeedPulseCount variable. Note the use of the volatile keyword. This tells the compiler not to optimise the use of this variable.

Next up is some support code. Two methods are initially required, ftoa adds the limited ability to convert a floating point number to a char * for debugging. The second method outputs a debugging message. This has been abstracted to allow for possible network debug messages later in the project. At the moment the serial port will be used.

TSL2561 – Luminosity Sensor

The next group of methods deal with the luminosity sensor, the TSL2561. these methods are slightly modified versions of the example code from Sparkfun. It is envisaged that future versions of these methods will deal with saturation and low light levels by dynamically changing the way the sensor works. For the proof of concept the basic code should suffice:

//
// Convert the error from the TSL2561 into an error that a human can understand.
//
void PrintLuminosityError(byte error)
{
switch (error)
{
case 0:
DebugMessage("TSL2561 Error: success");
break;
case 1:
DebugMessage("TSL2561 Error: data too long for transmit buffer");
break;
case 2:
DebugMessage("TSL2561 Error: received NACK on address (disconnected?)");
break;
case 3:
DebugMessage("TSL2561 Error: received NACK on data");
break;
case 4:
DebugMessage("TSL2561 Error: other error");
break;
default:
DebugMessage("TSL2561 Error: unknown error");
}
}
//
// Set up the luminsoity sensor.
//
void SetupLuminositySensor()
{
light.begin();
// Get factory ID from sensor:
// (Just for fun, you don't need to do this to operate the sensor)
unsigned char id;
if (light.getID(id))
{
sprintf(buffer, "Retrieved TSL2561 device ID: 0x%x", id);
DebugMessage(buffer);
}
else
{
byte error = light.getError();
PrintLuminosityError(error);
}
// The light sensor has a default integration time of 402ms,
// and a default gain of low (1X).
// If you would like to change either of these, you can
// do so using the setTiming() command.
// If gain = false (0), device is set to low gain (1X)
// If gain = high (1), device is set to high gain (16X)
gain = 0;
// If time = 0, integration will be 13.7ms
// If time = 1, integration will be 101ms
// If time = 2, integration will be 402ms
// If time = 3, use manual start / stop to perform your own integration
unsigned char time = 2;
// setTiming() will set the third parameter (ms) to the
// requested integration time in ms (this will be useful later):
light.setTiming(gain, time, ms);
// To start taking measurements, power up the sensor:
DebugMessage((char *) "Powering up the luminosity sensor.");
light.setPowerUp();
}
//
// Read the luminosity from the TSL2561 luminosity sensor.
//
void ReadLuminositySensor()
{
unsigned int data0, data1;
if (light.getData(data0, data1))
{
sprintf(buffer, "TSL2561 data: 0x%04x, 0x%04x", data0, data1);
DebugMessage(buffer);
//
// To calculate lux, pass all your settings and readings to the getLux() function.
//
// The getLux() function will return 1 if the calculation was successful, or 0 if one or both of the sensors was
// saturated (too much light). If this happens, you can reduce the integration time and/or gain.
// For more information see the hookup guide at:
// https://learn.sparkfun.com/tutorials/getting-started-with-the-tsl2561-luminosity-sensor
//
//
// Perform lux calculation.
//
double localLux;
good = light.getLux(gain ,ms, data0, data1, localLux);
if (good)
{
lux = localLux;
}
}
else
{
byte error = light.getError();
PrintLuminosityError(error);
}
}
//
// Log the luminosity data to the debug stream.
//
void LogLuminosityData()
{
sprintf(buffer, "Lux: %s", ftoa(number, lux, 2));
DebugMessage(buffer);
}

BME280 – Air Temperature, Pressure and Humidity Sensor

Dealing with this sensor is simpler than the luminosity sensor as can be seen from the code below:

Note that this sensor does allow for an enable line. This can be used to disable the sensor and put it into a low power mode if necessary. This will not be used in the proof of concept will be considered when the project moves to the point where power supplies / solar power is considered.

Wind Speed Sensor Interrupt Service Routine (ISR)

The non-trivial thing to remember about this code is that the method changes a global variable. The volatile keyword used in the variable definition is necessary to stop the compiler from optimising the global variable as this can have side effects.

Data Logging to Sparkfun’s Data Service

Assuming that we have a WiFi connection then we can log the data collected to the Sparkfun data service (this is based upon Phant).

The stream has been set up to collect more data than is currently collected, for instance, ground temperature. Any parameter not measured at the moment is set to 0.

Some other things to consider following the proof of concept:

Error handling for network issues

Possible offline collection of data

Using a local version of Phant

Something to bear in mind after the project moved from proof of concept.

Data Collection

The majority of the sensor have a read method to collect the data from the sensor. The only exception at the moment is the wind speed sensor. The data collection is performed inside the main method for collecting and logging the sensor readings:

At the start of setup we need to rely upon the Serial object being available and so there are no calls to the DebugMessage method as this may be modified later to use networking for debugging. The next step is to try and connect to the network:

At this point the application will be either looping indefinitely until the network becomes available or we will have an IP address output to the serial port. In a more complete application this will start the logging process and periodically try to connect to the network when the network is unavailable. This is only a proof of concept after all.

There is currently no Real Time Clock (RTC) attached to the system and so we need to check the network time at startup.

//
// Get the current date and time from a time server.
//
DebugMessage("Setting time.");
ntp = ntpClient::getInstance("time.nist.gov", 0);
ntp->setInterval(1, 1800);
delay(1000);
ntp->begin();
while (year(ntp->getTime()) == 1970)
{
delay(50);
}

This block of code loops until the network time is set correctly. At start up the year will be set to a default value of 1970, this is why the code loops until the year is something other than 1970.

The current public data stream for this service should be viewable here.

Checking this stream you can see the resulting data that was sent to Sparkfun for the above serial output:

Phant Data From Weather Station Proof of Concept

One thing to remember when comparing the above data is that the Sparkfun data stream logs data using UTC and the application above was running during British Summer Time.

Conclusion

The code above is a proof of concept, it is not error proof nor does it take into account power usage or optimisation, it is merely meant to prove that the sensors can be read and the data output to a cloud service.

Next steps for the proof of concept:

How to handle two sensors requiring an analog conversion

The project is running out of digital pins

Real time clocks

Offline data logging

All of this before even looking at the location and powering the project in the wild (OK, my garden).

The Ultraviolet light and wind direction sensor will present us with a problem as they are both analog sensors and the Oak only has one Analog-to-Digital Convertor (ADC).

The rainfall gauge and the wind speed sensor both use a similar technology to generate a signal, namely a magnet that will trigger a reed switch. These two sensors also present an issue; while they are simple enough there are a finite number of pins on the Oak and they are being consumed at a fair rate.

ML8511 – Ultraviolet Light Sensor

The ML8511 measures ultraviolet light at a wavelength of 365nM. This is at the top end of the UVB band, the band which is harmful to living tissue. The sensor generates a voltage that is linear and proportional to the intensity of the UV light. The intensity of the light measured is on the scale 0 to 15 mW/cm2. The following chart is taken from the data sheet:

ML8511 UV Plot

The chart above is generated when the supply voltage is 3.0V but the system under development will be using a 3.3V supply. Some investigation is required to determine the output voltage when there is no UV present. After running the sensor under constant temperature conditions with no UV light present the ADC was generating a reading in the 316 – 320 range. This gives out output voltage for the sensor in the range 1.02V – 1.03V.

If we assume that the output characteristics of the sensor remain linear at 3.3V and the gradient remains the same then we can generate a formula for calculating the intensity of the UV light based upon the sensor output.

All linear graphs can be represented by the following formula:

y = mx + c

Where:

y is the y coordinate (in our case the voltage output of the sensor)

x is the x coordinate (in our case the intensity of the UV light)

m is the gradient of the line

c is a constant offset of the y coordinate

If we set x to 0 then the equation becomes y = c This represents the output of the sensor when there is no UV light present. As we have seen from the above experiment, this is in the range 1.02V – 1.03V. So as an approximation we will use the value c = 1.025V.

The gradient of any line is represented by the following formula:

m = deltaY / deltaX

When given two points on a line (x1, y1) and (x2, y2), then the gradient becomes:

m = (y2 – y1) / (x2 – x1)

Luckily the data sheet gives us two points on the line at 25C, namely the output voltage in shade and the output voltage at 10mW/cm2. So assuming shade represents no ultraviolet light then the gradient of the line becomes:

m = (2.2 – 1.0) / (10 – 0)

So m is 0.12. Plugging m and c back into the original equation gives:

y = 0.12x + 1.025

Solving for x gives:

x = (y – 1.025) / 0.12

Or to put it in context:

UV Intensity = (Sensor output in volts – 1.025) / 0.12 mW / cm2

There are some assumptions in the above work and the data sheet shows that the output voltage can vary but we have a method for calculating the intensity of UV light; accurate enough for a home project anyway.

Wind Speed

Wind speed is measured by an anemometer. The anemometer in the Weather Sensor kit is a cup anemometer:

Cup Anemometer

This has a magnet on the spindle connected to the cups. The magnet closes (and opens) a reed switch each time it passes the switch. So one pulse is generated per full revolution of the spindle. Each full revolution of the spindle (per second) represents a wind speed of 1.492 miles per hour (mph) or 2.4 km/h.

For the experienced, this sounds simple but we all know there could be a nasty shock in store for us, namely switch bounce. An easy way to find out, hook the output up to an oscilloscope.

The circuit is simple enough, connect one switch contact to 3.3V, one to a resistor and the other end of the resistor to ground. Connect the scope to the resistor / switch junction.

The scope was set up to have a trigger voltage of about 1.5V and to trigger on the falling edge of a signal. The cups on the anemometer were then position so that the reed switch was closed (a high output on the scope) and the scope setup in single shot mode (to capture and hold the trace when triggered). Spinning the anemometer gave the following output:

Anemometer Switch Bounce

As you can see, the switch does bounce. Switch debouncing is a well known and documented problem, in fact I have written about it here so we will not go too deep into the problem in this article. The solution that will be used is a simple RC circuit:

MSP430 Launchpad Debounce Circuit

The principle is that the RC circuit resists change and so filters out the glitch in the above trace. So we need a filter that is resistive enough to filter out the glitch but fast enough to respond in the minimum time between pulses.

The weather station is going to be located in the mainland UK, about 30 miles from the coast. In this location the wind speed is unlikely to rise above 60-80 mph unless in extreme conditions (tornadoes are known to occur in the UK). So assuming the maximum wind speed in 149.2 mph (this number is based upon the fact that 1.492 mph gives one revolution per second) then we have a maximum number of rotations of 100 per second. This gives a revolution time of 10 milliseconds.

So we have a 10 millisecond window for the pulse from the sensor. The pulse will be low for the majority of the time as the switch can only close when the magnet is above the switch, so for most of the 10ms we will have a low pulse. You can see this in the following trace:

Anemometer Duty Cycle

The signal appears to be high for 30% of the time. An accurate measurement could be made using the scope but it does not appear to be necessary. In our case, 10 milliseconds per rotation, the signal would be low for approximately 70% of the time, i.e. 7 milliseconds.

Going back to the first trace should the switch bounce it is observed that the switch bounce lasts for approximately 50-60 microseconds. Several observations of both the rising and falling edges showed this to be reasonable consistent. This final piece of information helps to define the parameters for this problem:

The frequency of the pulses should be 100Hz maximum (i.e. 10 milliseconds between pulses)

Duty cycle is 30% (high for 3 milliseconds, low for 7 milliseconds)

Switch bounce can last 50-60 microseconds (assume 100 microseconds as a worst case)

Trigger voltage is 2.3V with a supply voltage of 3.3V

There are plenty of online calculators for this type of circuit, Layada has one here that covers this type of scenario. We know the supply voltage (3.3V) and the trigger voltage (2.3V) so the only thing to do is look at the components available and calculate the delay time. After a few tries a 10K resistor and a 100nF capacitor were found to give a delay of 1.1939 milliseconds. This covers our case with plenty of margin for error.

Putting together the circuit above where the anemometer is the switch and triggering on the rising edge gives the following output on the oscilloscope:

Anemometer Gradual Rise

The rise time looks to be acceptable, smooth and slow enough to iron out the glitches but fast enough to allow a 10 millisecond duration with a 30% duty cycle.

Conclusion

At the start of the article it was noted that four analog sensors are present in the kit. This article has concentrated on just two, the UV and Wind Speed sensors. The Rainfall and Wind Direction sensors will be covered in future articles.

The two sensors here will require one digital pin and one analog pin. As there is only one analog pin on the Oak then some creative thinking will be required in order to connect the Wind Direction sensor.

Assuming a maximum wind velocity of 149.2 mph then we will have 100 rotations of the anemometer per second. This should be something that could be measured by the Oak using an interrupt on one of the digital pins

At this point the hardware for six measurements can be put together on breadboard and some prototype software put together. This is the subject of the next article, Proof of Concept Software

The weather station project will be bringing together a number of sensors, light, ultraviolet light, air pressure, humidity, wind speed, wind direction and rain fall. This collection of sensors falls into three groups:

Electronic sensors on an I2C bus

Mechanical sensors using switches

Analog sensors

The current plan is for the weather station to use the Oak as the microcontroller running the show. The data from the sensors can then be uploaded to the cloud, destination to be determined, but let’s start with Sparkfun’s data service.

The I2C sensors will require the least amount of work to get up and running so let’s start with those. The two sensors operating on the I2C bus are:

One of the great things about working with these two sensors is the fact that there are prebuilt drivers and example code for both breakout boards available from Github. What could be simpler, well head over to the Sketch – Include Library – Board Manager… menu in the Arduino IDE and you can download the library and have the IDE install it for you.

TSL2561 – Luminosity Sensor

This sensor allows the radiance of the light to be calculated in a way that approximates the response of the human eye. It does this by combining the input from two photodiodes, one infra-red only and one visible light and infra-red light combined. The output from the two sensors can be used to luminous emittance in lux (lumens per square metre).

The following table gives an idea of the lux values for typical scenarios:

Lux

Typical Environment

0.0001

Moonless, overcast night sky

0.002

Moonless clear night sky

0.27–1.0

Full moon on a clear night

80

Office building hallway

320–500

Office lighting

1000

Overcast day

10000–25000

Daylight

As you can see from the table above, the lux values for a “normal” human day can vary dramatically. The sensor copes with this by allowing the use of a variable time window and sensitivity when taking a reading. Effectively the sensor accumulates the readings over the time window (integration interval) into a single 16-bit number which can then be used to calculate the lux reading.

BME280 – Air Pressure, Temperature and Humidity Sensor

This sensor is produced by Bosch and is packaged in both I2C and SPI configuration on the same board. The accuracy of the sensor appears good, pressure and temperature both to 1% and humidity to 3%.

Libraries

Both Sparkfun and Adafruit have provided libraries and example code for the boards. These were easy to add to the development environment.

One caveat, the BME280 requires the addition of the Adafruit sensor library as well as the BME280 library.

Once added it was a simple case of wiring up the sensor to 3.3V and the I2C bus and running the example code.

They both worked first time.

Some Code Modifications (for later)

The light sensor has been show to work in low light conditions but not to any degree of precision. A possible modification to the example code is to look at the sensitivity and integration window settings to see if the precision can be adjusted to make the sensor return better readings in low light.

Some of the values when calculated use the fractional part of a floating-point number, temperature and humidity spring to mind. This meant adding a method to convert a double into it’s string representation for debugging purposes. Trivially solved but an annoying omission from the implementation of sprintf.

Posting to Particle Dashboard

The Oak can also post to the Particle dashboard providing a second method of debugging your application. Statements such as Particle.publish(“Debug”, “Temperature data…”); will cause the string Temperature data… to be posted in a group/attribute Debug

So the readings from the office look like this:

Some Weather Data

Conclusion

Two simple to use sensors with good supported class libraries make these sensors quick and easy to hook up to the Oak. Merging the two examples was simple and sensor data is now appearing over the serial output from the Arduino.

Compiling the code gives some warnings about the I2C library being compiled for the ESP8266 whilst the target board is defined as an Oak. This can be ignored as the Oak is really a convenient wrapper around an ESP8266 module.

The Oak microcontroller is new, Digistump only shipped version 1 firmware a few days ago (20th Match 2016). The hardware I have was shipped sometime in January. So the first thing to do is to upgrade the firmware and then try blinking an LED.

Upgrading the firmware

The Digistump Wiki contains a number of tutorials and troubleshooting guides. First stop the Connecting Your Oak for the First Time page. This shows how the Oak can be connected to your WiFi network and the firmware updated.

The initial over the air firmware update was problematic to say the least. Reading through the Digitsump forums it seems that I am not the only one having a problem with the first update. There are three methods for upgrading the firmware:

Over the Air using firmware from the internet

Over the air with a local server

Serial using pyserial or esptool

I started at the top of the list and slowly worked my way down. In the end the only method that worked for me was the serial update.

Flashing an LED

With the latest version of the firmware installed it is time to test the development environment. What could be simpler than flashing and LED. The on board LED is connected to pin 1 so lets try and use that.

At the time of writing there was a known issue with the Particle development environment which prevented an application being built and flashed successfully. This is an early release and so issues are expected.

There are two methods for flashing an application to the Oak using the Arduino environment:

Over The Air (OTA)

Serial (requires Python)

Despite the earlier problems with the firmware update I decided to try the OTA method first. DigitStump have provided compehensive instructions in their WiKi on how to achieve this.

Following the example was easy and after only 15 minutes I have the LED on the Oak flashing at 1Hz. Just to prove it was not a fluke I then tried changing the frequency and reflashing the Oak.

Success!

Conclusion

My initial frustration with the firmware update was soon forgotten once I had an application successfully running on the Oak. I am hoping that the issues were caused by the fact I have an early release of the board with the original firmware installed.

Programming is easy enough and can be done over the air which is convenient.

The first part of this year has seen me working on some photography projects, now time to get the soldering iron out. I have three projects I’m hoping to complete this year and one long term project that may take me a while, so pay attention 007.

Weather Station

A few weeks ago I received a couple of Oaks. These boards are based around the ESP12 module, a Kickstarter project I backed at the end of 2015. The project aimed to make working with the ESP12 easier by providing integration with particle.io. The latest firmware was release a few days ago so I think it is time to give this little device a go.

At the start of the year we came across a weather meter which allowed the measurement of rainfall, wind speed and wind direction. My wife mentioned that it might be nice to use one of these to record the weather in the garden.

Mapping Out The Project

After a few hours the project started to become larger than just a couple of sensors, it currently looks something like the following:

Weather Station Mind Map

I have coloured each block of concepts the same colour in order to give the project some structure. Let’s look at each block in turn and see how we can approach the problem.

Temperature, Pressure and Humidity

Starting with the green block and five o’clock on the diagram.

Temperature, Presure and Humidity Mind Map

There are two temperatures we can measure here, air temperature and soil or ground level temperature. The air temperature will give an indication of how pleasant a day it is at the moment and the ground level sensor will give an indication of the progress of the seasons from the plants perspective. For this reason we are interested in both of these measurements.

Air pressure has long been used as a predictor for upcoming weather events.

Luckily we can get the air temperature, pressure and humidity sensor in one convenient package, the BME280. Ground level temperature will need a waterproof sensor. I think this would be best purchased as a unit rather than made so the DS18B20 with a 6 foot cable looks like it might be ideal.

Wind and Rain

Working clockwise, the next block are the wind and rain sensors:

Wind and Rain Sensor Mind Map

All of these measurements come from the weather meter. The wind speed and rainfall sensors are simple switches that generate pulses whilst the wind direction is a resistor network.

Light Sensor

The next block shows the two light sensors:

Light Sensor Mind Map

The two light measurements are the overall light intensity (luminosity) and ultraviolet light intensity. Luckily there are a couple of sensors for these two measurements, the TSL2561 and the ML8511.

The ultraviolet light sensor is an analogue sensor and so we will have to consider the stability of the supply voltage when making the reading.

The luminosity sensor uses a measurement window and a sensitivity setting to take a reading. This means that for given settings the sensor may be overwhelmed and simply give a maximum reading. The work around for this is to make the measurement window and sensitivity dynamic. So long, sensitive windows at night and short less sensitive windows on bright sunny days.

Cases and Location

The next things we need to consider are cases and locations:

Case and Location Mind Map

The microcontroller and power etc. will need to be located in a case of some form. This will need to be weather proof as water and electricity are not the best of friends. It would also be a good idea to keep any batteries in an environment with a reasonably stable temperature. A little research into how the professionals house weather Stations.

The sensors on the other hand need to be outdoors in a suitable location for the measurements being taken.

Power Supply

All of this equipment will need a stable power supply:

Power Supply Mind Map

The initial work can be done using a bench power supply but when the project moves outdoors it will need to either be mains or battery powered. The long term aim is to use a solar cell and rechargeable battery, as they say on Kickstarter, a stretch goal.

Data Logging and Real Time Clock

Now we have collected all of this information we need to do store it somewhere:

Data Logging Mind Map

The Oak is a WiFi enabled board so the most obvious place to put the data is the cloud. It might be an idea to also provide some local storage in case the WiFi network is unavailable.

The Real Time Clock (RTC) could have two uses, to wake the microcontroller and also provide a timestamp for the data items.

Conclusion

What started out as a simple wind and rain logging project has grown a little. The current specification is to measure and log the following readings:

Rainfall

Wind speed

Wind direction

Luminosity

Ultraviolet light

The measurements should be collected and reliably logged either locally or preferably to the cloud.

There are several challenges with a number of interesting problems to overcome.

A few weeks ago I started to work with KiCAD and put together a small LED board as a project to become familiar with the application. The boards were sent to manufacture and the the postman delivered the manufactured PCBs yesterday.

LED Board Front

LED Board Front

Time to add the components and test the boards.

Populated Board Right Angled Headers

And one with straight headers:

Populated Board Straight Headers

Everthing looks good, the boards work OK so time to move on to the big board for the cube.